1. Field of the Invention
This invention relates to a process for injection molding a preform of a polyester resin, especially a blend of polyethylene terephthalate and polyethylene naphthalate as a molding material, to an injection screw therefor, and to a process for stretch blow molding a hollow molded article such as a thin-wall vessel from the preform.
2. Background Art
Since a stretch blow molded article of polyethylene terephthalate (to be abbreviated as "PET" hereinafter) has good transparency, good impact resistance and relatively good gas barrier property, it is widely used in drink bottles. As for heat resistance, it has a thermal deformation temperature (glass transition temperature) of about 75.degree. C. and hence is subjected to a heat treatment such as heat setting to improve its heat resistance when it is used in drink bottles for juice and natural water that must be filled at high temperatures (around 80.degree. C.). For this reason, a PET bottle called "heat resistant bottle" is more expensive than an ordinary PET bottle which is commonly used.
Polyethylene naphthalate (to be abbreviated as "PEN" hereinafter) which is a polyester resin like PET has a melting point of around 270.degree. C. and a glass transition temperature of around 124.degree. C., which are higher than the melting point and glass transition temperature of PET in spite that they are both a polyester resin. Moreover, PEN has satisfactory heat resistance when used as a thin-wall vessel and excellent gas barrier property and transparency when it is stretched as well as strength.
In this way, PEN is more suitable than PET for use in a drink bottle or a wide-mouthed vessel that requires high-temperature filling and gas barrier property although they are both a polyester resin. However, the material costs of PEN are higher than those of PET and a PEN bottle is much more expensive that a heat set PET bottle, thereby making it difficult to use PEN at the moment.
To solve the above problem, attempts have been made to develop a thin-wall polyester bottle having excellent heat resistance and gas barrier property which is formed from a blend of PET and PEN as a molding material. According to existing materials on heat resistance, a stretch blow molded bottle formed from PET as a substrate can withstand a temperature of 83.degree. C. when it is blended with 10% of PEN and its heat resistance can be further improved by increasing the amount of PEN blended.
However, PET and PEN have weakness in compatibility. When they are molten and mixed together, the resulting mixture becomes semi-opaque and turbid. It is considered that this turbidity is caused by the formation of two phases by PET and PEN in a molten state and is distinguished from a whitening phenomenon caused by crystallization. However, this opacifying phenomenon can be eliminated when an ester exchange reaction is promoted during the plastication of a molding material to change the material into a copolymer at the time of injection molding. Thereby, a transparent molded article, i.e., a preform can be molded.
For the purpose of promoting and controlling an ester exchange reaction in this molten state, Amoco chemical Co. of USA provides a PEN material which is a copolymer composition containing 8.0 mol % of terephthalate and 92 mol % of 2,6-naphthalene dicarboxylate (called as "NDC"). It is said that use of this copolymer composition can reduce the crystal melting temperature (267.degree. C.) of PEN to the same temperature as the crystal melting temperature (250.degree. C.) of PET with the result of an increase in the contact time between PET and PEN contained in the blend in a molten state, thereby making it possible to obtain a preform in which an ester exchange reaction is promoted. However with regular injection molding which has been employed so far, it has been difficult to mold a transparent preform.
To promote an ester exchange reaction in a heating cylinder in order to mold a transparent preform, an attempt is being made to carry out plastication by setting the temperature of the heating cylinder to 310.degree. C. or more and back pressure to a high level. It is assumed that, since melting and kneading are promoted at such a high temperature and a high back pressure and residence time is prolonged, phase separation is eliminated, and the molten material is injected in the state of a copolymer due to the promotion of an ester exchange reaction, thereby making it possible to obtain a transparent preform.
However, since the residence time of the molten material in the heating cylinder is inevitably long and the material stays in the heating cylinder at a high temperature and at a high pressure, the molten material is excessively heated and easily decomposed, and even partly gasified. This excess heat has a bad influence upon the physical properties of the polyester resin, causing discoloration, burning, deterioration and the like disadvantageously. In addition, metering time is also prolonged and accordingly, the molding cycle is naturally prolonged. Especially, in a blend of PET and PEN, PET is affected by excess heat and decomposed because the melting point of PET is lower than that of PEN, thereby increasing the amount of acetaldehyde produced.
A blend of PET and PEN has another problem to be solved in addition to its opacity. This problem is that a small gel-like granular material called "fish eye" is produced. Although this granular material does not have a great influence upon the shape of a preform, it remains on the thin-wall body of a final bottle product as a flaw, causing a defective product.
Therefore, to obtain a thin-wall bottle from a molding material which is a blend of PET and PEN and has improved heat resistance by stretch blow molding, it is necessary to prevent a preform from becoming opaque at the time of injection molding and the occurrence of a fish eye. Otherwise, it is impossible to obtain a hollow molded article of a polyester resin such as a thin-wall bottle or a wide-mouthed vessel which makes use of the physical properties of PET and PEN.